For many internal combustion engines, their engine control strategy has three important parameters: spark timing (or fuel injection timing in a diesel engine), the exhaust gas recirculation (EGR) rate and the air/fuel ratio (AFR). To implement EGR, a fraction of the exhaust gas is recycled from the exhaust system back to the intake system. The recirculated exhaust gas is mixed with the fresh fuel-air mixture before entering the cylinders.
EGR has a long history of use in both diesel and spark-ignited engines for reduction of NOx emissions. It affects combustion in several ways. The combustion is cooled by the presence of exhaust gas, that is, the recirculated exhaust gas increases the charge heat capacity. The dilution of the oxygen present in the combustion chamber reduces the production of NOx, primarily by reducing the adiabatic flame temperature.
One approach to implementing EGR is with one or more dedicated EGR cylinders. In a “dedicated EGR” engine, the one or more dedicated cylinders are used to generate exhaust gas, all of which is dedicated to recirculation. The dedicated EGR may be produced by a cylinder having a single exhaust port that opens only to an EGR loop. The other cylinders produce “normal” exhaust, i.e., exhaust that exits the engine via one or more exhaust aftertreatment devices. U.S. Pat. No. 8,291,891, entitled “EGR System with Dedicated EGR Cylinders”, to Alger, et al., and U.S. Pat. No. 8,561,599 entitled “EGR Distributor Apparatus for Dedicated EGR Configuration”, to Gingrich et al., both describe systems that generate dedicated EGR in this manner and are incorporated by reference herein.
A feature of dedicated EGR is that the composition of the dedicated EGR exhaust gas may be controlled to be different from that of the exhaust of the non-dedicated cylinders. For example, in a gasoline engine, the dedicated EGR cylinder(s) may be run rich to provide EGR that improves combustion on all cylinders. With dedicated EGR, the quality of the recirculated exhaust can be improved by increased in-cylinder reforming of gasoline to H2 and CO. Subsequent combustion of this exhaust by the engine is thereby enhanced.